First off, height doesn't matter except in rare circumstances, so I say the height should be whatever you feel like.
As for the width:
About 600 pixels wide is fairly common and fits well with most webpage designs, forum posts, deviantart's defaults, drunkduck, etc. You can go to about 650 with decent readability on just about any device and any website.
Others go with 800-900 pixels-wide, which is great for anyone with a decent monitor, but a little tough on mobile gadgets. It also may cause you to run into resizing issues on drunkduck and in forum posts... but if you're uploading strictly to your own website, it shouldn't be a problem.
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800x300 works wonders thanks!
Well, the beauty of digital comics is, There's no standard size! try to find a size that fits what you're doing, tweak the dimensions for effect, if you want, try using infinate canvas. look up scott mccloud's book making comics. he knows what he's talking about, and every comic writer/atist/anything should read it before they get started. trust me
I may best answer you but I'll wait a little longer for more.